Protective surface coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other additives, where the polymeric binder acts as a fluid vehicle for the pigments and imparts rheological properties to the fluid paint coating. Upon curing, the polymeric binder hardens and functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness, although some paint coatings contain little or no opacifying pigments and are described as clear coatings. The manufacture of paint coatings involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in the polymeric binder, and thinning to commercial standards.
Epoxy resins are particularly desirable for use in surface coating materials as a vehicle or polymeric binder to advantageously provide toughness, flexibility, adhesion, and chemical resistance to the applied coating film. Hence, water-dispersed coating compositions containing epoxy resins are highly desirable for can coating compositions. Coatings for soft drink and beer cans, for instance, are critical due to taste sensitivity and must not alter the taste of canned beverages. Taste problems can occur in a variety of ways such as by leaching of coating components into the beverage, or by adsorption of flavor by the coating, or sometimes by chemical reaction, or by some combination thereof.
In commonly assigned U.S. Pat. No. 4,212,781, a process is disclosed for modifying epoxy resin by reacting the epoxy resin with addition polymerizable ethylenic monomer in the presence of at least 3% by weight of benzoyl peroxide (or the free radical initiating equivalent thereof) based on monomer at a suitable reaction temperature to produce a reaction mixture comprising an epoxy-acrylic copolymer mixture containing epoxy resin, graft epoxy-acrylic polymer, and associatively-formed ungrafted addition polymer. The in-situ polymerized monomers include acid functional monomers to provide acid functionality in the reaction mixture sufficiently high to effect stable dispersion of the resulting reaction product in a basic aqueous medium. Similarly, commonly assigned U.S. Pat. No. 4,522,961 pertains to a mixture of polymers comprising a self-curing emulsion polymer (latex), an epoxy-acrylic graft copolymer, and preferably a phosphate additive. Related commonly assigned patents are U.S. Pat. Nos. 4,285,847 and 4,399,241, and 5,212,241. Most prior art water-dispersed epoxy coatings utilize relatively high levels of organic solvent to assist processing of the epoxy resin. Although epoxy containing coatings have long been the standard of excellence in beverage can liners, such coatings cannot be prepared without significant amounts of solvent, where typically 50% to 100% volatile organic solvent is required based on solids (about 2.5 to 4 lb./gal.). Recent environmental concerns and legislative pressure have created the need for a zero or near zero VOC can coating.
It now has been found that high quality aqueous dispersed epoxy coatings can be produced with no organic solvent by esterifying low molecular weight epoxy resin with low molecular weight carboxyl functional polyester to produce a carboxyl functional low molecular weight epoxy-ester, mixing the epoxy ester with ethylenic monomers and dispersing the mixture within water, and then copolymerizing the ethylenically unsaturated monomers to produce an emulsion polymer useful as a polymeric binder. In particular, it has been found that aqueous emulsion dispersions of the epoxy-ester addition copolymer can be prepared and maintained dispersed in water without conventional surfactants and by ammonia neutralization of the epoxy-ester carboxyl groups without the need for any volatile organic solvents. Copolymerization of the ethylenic monomers produces a stable small particle size polymeric dispersion.
It has been found that low molecular weight epoxy resins are easily processable at lower temperatures and viscosities while low molecular weight oligomer carboxyl functional polyesters lower the overall viscosity of the mixture and provides considerably improved processability. Temperature control during the formation of the epoxy ester advantageously avoids unwanted molecular weight advancement while the liquid ethylenic monomers serve as a temporary solvent for the epoxy-ester which in turn facilitates the simple dispersion of the organic mixture into water. Once epoxy resin is reacted with a low molecular weight, acid functional polyester oligomer in accordance with the invention, the resultant epoxy ester can be dissolved in acrylic monomer and dispersed into water with very low levels of ammonia. The epoxy-ester is water dispersed and becomes addition grafted (crosslinked) with the copolymerized ethylenic monomers to form a very small particle microgel stably dispersed in water. Grafting of the epoxy-ester with ethylenic monomers in water produces very small size crosslinked microgel particles, a physical property particularly useful for producing tough but resilient and flexible coatings.
This invention incorporates the advantages of epoxy chemistry providing good barrier properties and excellent resistance to flavor absorption along with a synthesis technique that eliminates the need for organic solvents, organic amines, and surfactants. Baked paint films utilizing the resulting polymeric binder are clear, glossy, solvent resistance, and water resistant. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples.